Starch carboxymethylation by reactive extrusion: Reaction kinetics and structure analysis.

An efficient reaction of carboxymethylation using reactive extrusion (REX) on plasticized starch was studied. The reaction products were characterized by Nuclear Magnetic Resonance (NMR) spectroscopy and high-performance liquid chromatography (HPLC). Several parameters affecting the reaction were investigated including the amount of plasticizer, reagent stoichiometry and reaction time. Reaction efficiency (RE) up to 65% was achieved in one step after 5 min of reaction, showing that the reaction kinetics is hundred times faster than the same reaction described in solution. The degree of substitution (DS) of 2.1 was obtained in 3 steps. The regioselectivity of the reaction on the anhydroglucose monomers units (AGU) was respectively C-2 > C-6 > C-3. A significant deviation from the Spurlin statistical model was observed. Multisubstituted AGU were analyzed by chromatography. Different multi-substitution patterns were obtained by using different starch sources. Tetra-carboxymethyl glucose corresponding to the complete modification of the terminal non-reducing glucoses from branched amylopectin chains was analysed.